Northern AZ wind and sun 420W RV system install

Re: Northern AZ wind and sun 420W RV system install

oldgearhead wrote: »

New guy here...I finally got my system from NAZWS and getting ready to install. But being in blog, amp, volt, wire size overload, (yes there is a ton of info and a lot of opinions out there). The wiring diagram on this site is great for for guys like me (great pictures). The sys. includes 3 kd140sx panels, Morningstar mppt 45 amp controller, baby box with 15 and 50 amp breakers and I purchased the remote ts-rm-2 display. The system came with #10 wire and of course install manuals in more languages than I can read. 12v dc is nothing new to me as my blog name hints (Oldgearhead), I have wired more cars and toys than I can count and wire size and volt drop are two items in my top 5 in wiring. My runs in my system will be 18 feet from panels to controller and less than 5 ft from controller to batteries. This is something I only want to install ONCE as I hate redo's. I own full shop and wire of any size is no problem on install, up-sizing in most cases is good and I do not mind but I only have room for one more panel on my roof at a later date. Sooo the questions are

#1 Is # 10 wire for 16 to 20' of travel from panels to controller enough

First, the solar panels... The 140 Watt Kyocera are Imp=7.91amps/Vmp=17.7 volts. Using a simple voltage drop calculator for 20' and 3% maximum drop (3% of 17.7 volts = 0.5 volt drop)--1% is better (less lost power), but going less than 1% will cost you a lot of money copper wiring:

• 20' @ 7.91 amps and 0.5 volt drop => 10 AWG @ 0.4 volt drop (each panel home run to controller)
• 20' @ 7.91*3= amps 24 and 0.5 volt drop => 6 AWG @ 0.5 volt drop (bus panels together and one run to controller)

Note, that when you have three or more solar panels in parallel, you should have a series protection fuse in series. 15 amps for this panel (PDF Spec)

#2 In there install directions no mention of wire size so what would be the correct size from controller to batteries ?

There should be... But very simply, you need (at a minimum) of 1.25x the maximum continuous current available from the controller. Roughly 3*Isc (short circuit) for those panels will be 3*8.68 amps = 26 amps. Using an NEC wiring Table we would see a minimum of 12 or 10 AWG (depending on insulation) wire.

Wire Current Ampacities NEC Table 310-16

Next, you want to look at the voltage drop... For most controllers, on a 12 volt battery bank I would suggest a maximum drop of 0.05 to 0.10 volts... So 26 amps at 5':

• 26 amps @ 5' and max 0.10 volt drop => 4 AWG with 0.8 volt drop

Now, that is a lot of copper--So if you can shorten the cable by 1/2, you could get up to 6 or 8 awg.

Also, with the MorningStar TSW family of charge controllers--They have a neat option--You can run a pair of small awg cables from the controller to the battery bank--Remote Voltage Sense Wires. This will allow you to use a bit smaller awg wiring (and more losses) but still have very accurate battery voltage measurements.

12 volt system especially, are a game of 1/10ths of volts--You want no more than ~0.5 to 1.0 volts or so maximum drop from your battery bank to the load/AC inverter... More drop, and your inverter/loads may cutout under heavy loads (10.5 volt inverter cutoff+1.0 volt drop = 11.5 volts--More or less a somewhat discharged battery bank under heavy load).

#3 In the baby box 1 15amp to controller and a 50 amp from controller to batteries. I understand the no add on amps in series but 50 amp from controller to batts. Hummmmmm

What are your loads--those drive the sizing of the wiring (and battery bank too). For example, a very nice 12 VDC TSW 300 watt/600 watt 10 minute surge inverter from MorningStar (did I say, very very nice?) would require a branch circuit capable of:

• 600 watts * 1/10.5 volts cutoff * 1/0.85 inverter eff * 1.25 NEC derating for wiring/fuses/breakers = 84 amp rated branch circuit

Of course, if you are not planning on using 600 watts, but only 300 watts--you could cut that branch circuit rating by 1/2.

One last question what size battery bank would accommodate this sys. without overkill as I live in Arizona and boondocking is a way of life and being retired with the time helps.

Normally, I would ask you want your daily loads are... 1,000 WH (1kWH per day or ~ 83 AH at 12 volts) per day is a nice sized system for most people (lights, laptop/cell phone charging, water pump, etc.).

Or, we can size a system based on the size of the solar array... First, 5% to 13% minimum rate of charge for the battery bank--That would give us battery support for a 3*140w=420 Watt array of:

• 420 watt * 1/14.5 volts charging * 1/0.77 panel+controller derating * 1/0.05 rate of charge = 446 AH @ 12 volt max battery
• 420 watt * 1/14.5 volts charging * 1/0.77 panel+controller derating * 1/0.10 rate of charge = 223 AH @ 12 volt nominal battery
• 420 watt * 1/14.5 volts charging * 1/0.77 panel+controller derating * 1/0.13 rate of charge = 172 AH @ 12 volt minimum battery

If you could justify the size--I would suggest the 223 AH range of battery (2x 6 volt 220 AH "golf cart" batteries in series). That would give you a good panel to battery bank ratio (quick recharging in good weather). If you have alternate charging sources (genset, moving RV every few days with recharging from the vehicle alternator), you could go with the larger battery (or if weekend boondocking--recharge back at home).

Battery usage is around 1-3 days of storage and 50% maximum discharge. For RV folks, you don't have a lot of room for batteries+weight, and you may only camp on weekends (20-50x a year) vs full time off-gridders--So discharging your batteries deeper is not a sin (your batteries will age out instead of cycling out--Or if you replace a pair of batteries every two years--not the end of the world for a small bank).

Anyway, if you want 2 days of "no sun" and 50% maximum discharge (long life), the 223 AH @ 12 volt battery bank would support:

• 223 AH * 12 volts * 0.85 invter eff * 1/2 days * 0.50 max discharge = 569 WH per day
• 223 AH * 1/2 days * 0.50 max discharge = 56 AH @ 12 volts per day (DC Power, no AC inverter)

Using PV Watts for Flagstaff AZ, array mounted flat to roof, we get:

Month    Solar Radiation
(kWh/m 2/day)
1      3.09     
2      4.08     
3      5.16     
4      6.18     
5      7.03     
6      7.56     
7      6.56     
8      5.50     
9      5.60     
10      4.43     
11      3.34     
12      2.84     
Year      5.11      

I typically toss the bottom three months--Which leaves us February at 4.08 hours of "noon time equivalent" sun per day. A 420 watt array in Flagstaff (mounted flat to roof) will give around (20 year average):

• 420 Watt * 4.08 hours of sun * 0.52 system eff (including AC inverter) = 891 WH per day minimum for ~9 months of the year

I greatfully thank you to all who have contributed to this blog and the many hours of reading I have enjoyed, also to the great folks at Northern AZ wind and sun for there help and great site. PS they refunded some of my $$$ do to the drop in price of panels after my purchase...GREAT CUSTOMER SERVICE!!!!!!

NAWS has been great for us here on the forum--All of us here (other than the Admin Windsun) are volunteers here and pretty much run the forum as we believe is best for the community--The only request from NAWS was keep politics to a minimum and no spam/free ads (amazing how many southern California solar customers live in India, Bangladesh, etc.).

We would all love to see what your final system looks like--Post a thread with pictures.

-Bill